Chemical agent detecting and alarm devices are devices which have been developed for use by armed personnel in the event of chemical warfare to detect the presence of chemical agents or vapours and activate an alarm when the concentration of a detected agent exceeds a predetermined safe level.
The instruments are designed to subject an air sample to tests which detect the presence and concentration of certain chemical agents of interest and activate an alarm when the tests reveal an unsafe concentration. The devices are used in essentially two modes. In an automatic mode, they are used as standalone units at a desired site to continuously monitor ambient atmosphere at that site. A remote alarm may be connected to a device for communicating the alarm signal to another site. In a manual mode, they are carried by a user and manipulated to sample air immediately adjacent surfaces suspected of being contaminated. In order to conveniently facilitate the latter, the devices must be relatively light and portable.
While portable, many conventional devices tend to be sufficiently heavy and awkward that they cannot be conveniently and comfortably carried in one hand of a user and, thus, have tended to assume backpack configurations for use in the manual mode discussed above. Flexible tubes manipulated by the user extend from the backpacks for supplying air samples to the devices. Not only does this arrangement make the various meters on the instrument difficult for the user to read, the flexible tubes tend to absorb chemical agents which results in a reduction of both the response speed and sensitivity of the devices.
Some conventional devices utilize chemical tests to determine the identity and concentration of chemical agents in air samples. This places significant limitations on the number and type of chemical agents which can be monitored within the framework of a relatively light, portable device because the necessary apparatus and chemicals are space consuming and relatively heavy. In addition, the tests render the devices relatively complex in terms of design, structure, operation and maintenance. A particularly important consideration insofar as maintenance is concerned is the decontamination of devices which have been exposed to chemical agents. Chemical agents tend to display low surface tension properties and thus tend to migrate to sharp corners, joints and like discontinuities on exposed surfaces. Devices having relatively complex mechanical designs thus tend to be difficult to decontaminate.
A recently developed chemical agent detecting device overcomes many of the aforementioned drawbacks. Structurally, the device is extremely light and compact so that it can be comfortably and conveniently carried in one hand by a user. A rigid, tubular probe extends from one end of the device and serves as an inlet for admitting air samples into the device. Thus, the device is easily manipulated, the displays are readily accessible to the user and there is little, if any, absorption of chemical agents by the probe.
The device makes use of ion mobility principles to determine the identity and concentration of chemical agents of interest. In essence, the air samples drawn into the unit are ionized to produce primary positive and negative ions. The ions undergo complex ion molecule reactions which result in a mixture of ions and ionic clusters which are urged to a collector in the device. The differences in mobilities of the ions and ionic clusters result in different arrival times at the collector. The collector current as a function of time represents a mobility spectrum for the sample. Each peak is indicative of a particular chemical agent or species. The drift time indicates the identity of the species and the magnitude indicates the concentration of the species. After amplification, the collector current is fed to a microprocessor which determines the identity of the species represented by each peak on the basis of the drift time and the concentration of the species based on the height of the peak and causes this information to be output to a display.
While the latter device is highly suited for use in the manual mode, it is not particularly well suited for use in the automatic mode because it cannot be constructed to rest on a support surface in the proper position without hampering its use in the manual mode, it is fragile and therefore susceptible to damage if left unattended and cannot be provided with sufficient battery capacity to enable it to be used continuously for extended periods of time.